Considerable effort has been made to extend the practical resolution capabilities of positive tone development in immersion lithography from both materials and processing standpoints. One such example involves negative tone development (NTD), an image reversal technique allowing for use of the superior imaging quality obtained with bright field masks to print the critical dark field layers. NTD resists typically employ a resin having acid-labile groups and a photoacid generator. Exposure to actinic radiation causes the photoacid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in the resin. As a result, a difference in solubility characteristics in organic developers is created between exposed and unexposed regions of the resist such that unexposed regions of the resist are removed by the developer, leaving behind a pattern created by the insoluble exposed regions. Such a process is described, for example, in U.S. Pat. No. 6,790,579, to Goodall et al. For the resist chemistries described, the exposed areas of the resist layer can be selectively removed with an alkaline developer or, alternatively, the unexposed regions can be selectively removed by treatment with a suitable nonpolar solvent for negative tone development.
The inventors have observed that surface inhibition of the photoresist in NTD processes can result in “necking” of contact holes or “T-topping” in line and trench patterns in the developed resist patterns. This effect is illustrated in FIG. 1, showing a substrate 1, a layer to be patterned 2 and a patterned photoresist layer 3. During photoresist exposure, the polarity-switched region undesirably extends into regions 4 at the resist surface which lay beneath the edge portions of the opaque photomask pattern. This is believed to be a result of diffusion of stray light beneath edges of opaque mask pattern. During development with an organic developer, unexposed (unswitched) regions of the photoresist layer are removed to form contact hole pattern 5. The resulting pattern exhibits necking at the resist layer upper surface where the polarity-switched resist regions 4 are not removed. The occurrence of necking and T-topping generally results in a poor process window including depth of focus and exposure latitude. These problems can lead, for example, to random missing contact holes or to micro-bridging defects in the case of narrow trench or line pattern formation, thereby adversely impacting device yield.
U.S. Patent Application Pub. No. US2011/0294069A1, to Bae et al, discloses photoresist compositions that include a matrix polymer that is acid sensitive, a particular additive polymer that has a surface energy lower than a surface energy of the first polymer; a photoacid generator and a solvent. While that document recognizes problems associated with surface inhibition, further improved solutions to this problem are needed.
There is a continuing need in the art for improved compositions and photolithographic methods for negative tone development which allow for the formation of fine patterns in electronic device fabrication and which address one or more problems associated with the state of the art.